In this experimental study, we report on the mixing properties of interfacial colloidal floaters (glass bubbles) by chemical and hydrodynamical currents generated by self-propelled camphor disks swimming at the air-water interface. Despite reaching a statistically stationary state for the glass bubbles distribution, those floaters always remain only partially mixed. This intermediate state results from a competition between (i) the mixing induced by the disordered motion of many camphor swimmers and (ii) the unmixing promoted by the chemical cloud attached to each individual self-propelled disk. Mixing/unmixing is characterized globally using the standard deviation of concentration and spectra, but also more locally by averaging the concentration field around a swimmer. Besides the demixing process, the system develops a "turbulentlike" concentration spectra, with a large-scale region, an inertial regime, and a Batchelor region. We show that unmixing is due to the Marangoni flow around the camphor swimmers, and is associated to compressible effects.
We numerically study the dynamics of an ensemble of Marangoni surfers in a two-dimensional and unconfined space. The swimmers are modeled as Gaussian sources of surfactant generating surface tension gradients and are shown to follow the Marangoni flow filtered at their spatial scale in the lubrication regime, an unstable situation leading to spontaneous motion as soon as the Marangoni effect is intense enough. As the system is fully unconstrained, it is possible to study the various dynamical regimes from single swimmer, two-body interaction, to the many-particles case characterized by an efficient particle dispersion. We show that, although the present model is very simple, it reproduces the experimentally observed transition between a regime of dispersion by random agitation when the number of swimmers is moderate to the regime of crystallization with imperfect hexagonal lattice at high density.
Abstract. Under some conditions, a water droplet can bounce on a flat water surface. This paper contains a qualitative analysis of the bouncing phenomenon that leads to try to obtain drops with a nearly tangential incidence, the smallest radius and the highest velocity possible. We describe the experimental setup that we built, able to throw unique droplets at typical speed v $ 1 m.s
À1, radius R $ 4 Â 10 À4 m and impact angle u $ 15°w ith respect to the surface. Up to 4 bounces were experimentally observed. The experimental results' plot shows the statistical behaviour of the bouncing process: the initial conditions are not sufficient to predict the trajectory and consequently the number of bounces.
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